Over-voltage protection circuit

ABSTRACT

An over-voltage protection circuit is disclosed herein for protection against over-voltage of an energy storage device while charging. The circuit operates within the operational limits of a battery-operated device, such as a mobile or handheld device. The over-voltage protection circuit comprises an over-voltage protection device, and an over-voltage protection controller. The controller allows current to flow to the over-voltage protection device only when an energy storage device is experiencing over-voltage. In allowing current to flow to the over-voltage protection device only when the voltage across the energy storage device is above a predetermined voltage, power conservation is achieved.

This is a divisional of U.S. patent application Ser. No. 11/371,686filed on Mar. 9, 2006, which is a continuation of U.S. patentapplication Ser. No. 10/261,038 filed on Sep. 30, 2002 (now U.S. Pat.No. 7,035,070), which claims the benefit of priority from U.S.Provisional Application Ser. No. 60/325,551 filed on Oct. 1, 2001. Bythis reference, the full disclosure, including the drawings, of U.S.Provisional Application Ser. No. 60/325,551 is incorporated herein.

FIELD OF THE INVENTION

The present invention relates to energy storage components used inbattery-operated devices. In particular, the present invention relatesto protection against over-voltage of such energy storage componentswithin the operational limits of a battery-operated device whilecharging.

BACKGROUND OF THE INVENTION

Many mobile devices, such as cellular telephones, personal digitalassistants (PDAs), and other handheld computing and communicatingdevices, rely upon standard energy storage devices, such as batterycells, for providing power on which to operate.

Though disposable battery cells, such as alkaline cells, are awell-known and reliable technology, it is common in such mobile devicesto employ rechargeable battery cells. These rechargeable batteriesdepend on a number of known cell types, including Ni-Cad, Ni-MH, andLi-Ion cells. All these cells are known to those of skill in the art, asare some of their deficiencies.

Although some mobile devices are able to function with standardoff-the-shelf rechargeable batteries, many use a specialisedrechargeable battery made particularly for that make and model of mobiledevice. A charging device is necessary in order to recharge the mobiledevice's battery. Such a charging device may be a dedicated device, ormay be integrated into an existing accessory, such as a cradle. The lifeof the battery can be drastically curtailed by improperly charging, orover discharging the battery.

Over-voltage protection circuits are commonly used to prevent a voltageacross an energy storage device, such as a battery, from exceeding a setpredetermined, or threshold, voltage. Such an energy storage device cancomprise a plurality of energy storage components. Presently,over-voltage protection is typically achieved by connecting resistors inparallel with the energy storage device. In such over-voltage circuits,current continuously flows through the resistors whether the terminalvoltage is above or below the set predetermined voltage, resulting insignificant wasted power. Such a conventional configuration isillustrated in FIG. 1.

The energy storage devices 102, 104 illustrated in FIG. 1 are supercapacitors, showing an example of a particular energy storage device.However, those of skill in the art will appreciate, the energy storagedevices can be any suitable device, such as Ni-Cad, Ni-MH, and Li-Ioncells, for example.

FIG. 1 shows a typical over-voltage protection circuit that is wellknown in the art. In this circuit 100, energy storage devices 102, 104are connected in series. Each energy storage device has a parasiticinternal leakage current. The magnitude of the leakage current may varyover a range of values, even among energy storage devices from the samemanufacturing batch. These varying leakage rates result in the voltageacross different energy storage devices decreasing at different rates.When the energy storage devices 102, 104 are charged, the energy storagedevice with the lower leakage rate, and hence the greater voltage, canexceed the maximum voltage specified for that energy storage devicebefore the combined voltage of both energy storage devices reaches adesired terminal voltage. Resistors 106, 108 are placed in parallel withenergy storage devices 102, 104 respectively in order to equalise therespective voltage drops across each energy storage device. Chargingleads 110 are shown in the drawing, for connecting a charging circuit(not shown) to the energy storage devices.

As one skilled in the art can appreciate, the resistors act to increasethe total current flowing through each energy storage device, since theresistors are effectively in parallel with the parasitic resistance ofthe energy storage devices. This causes the energy storage devices todischarge any excess charge faster than if the resistors were notpresent. The resistor values are normally chosen so that the current ineach resistor is much greater than the largest specified internalleakage current of the individual energy storage device. Given that theresistors typically come from the same manufacturing batch and are quiteclosely matched in value (within a few percent), the rate at which thevoltage of the energy storage devices decrease is therefore more closelymatched than if the resistors were absent.

However, this configuration results in continually wasted power sincecurrent is constantly flowing through the resistors and the current ineach resistor is greater than the leakage current of the capacitor. Amore power-efficient solution is required.

It is therefore desirable to provide a configuration that allows currentto flow only when an energy storage component is above a predeterminedvoltage and thereby conserve power.

SUMMARY OF THE INVENTION

It is an object of the present invention to obviate or mitigate at leastone disadvantage of previous over-voltage protection circuits,particularly those provided for use with handheld or mobile devices.

In an aspect of the invention, a protection circuit to preventover-voltage of an energy storage device while being charged isprovided. The energy storage device can be, for example, a supercapacitor, or a lithium-ion battery.

In a first aspect, the present invention provides an over-voltageprotection circuit for connection to a charging circuit for maintaininga voltage across an energy storage device at or below a predeterminedvoltage during charging. The circuit comprises an over-voltageprotection device and an over-voltage protection controller. Theover-voltage protection controller connects the over-voltage protectiondevice in parallel with the energy storage device only when the voltageacross the energy storage device exceeds the predetermined voltage, soas to draw excess charge from the energy storage device.

The over-voltage protection controller can comprise a switch actuated inresponse to an over-voltage condition at the energy storage device. Thecontroller can further comprise an over-voltage detector coupled to theenergy storage device and to the switch, which causes the switch to beactuated when a voltage measured across the energy storage deviceexceeds the predetermined voltage. This over-voltage detector and switchcan be integral with one another.

In general, the over-voltage protection device either dissipates excesscharge drawn from the energy storage device, or temporarily stores theexcess charge.

A resistor is an example of an over-voltage protection device thatdissipates excess charge drawn from the energy storage device. A zenerdiode or a shunt resistor may also be used as a dissipating over-voltageprotection device, with the added advantage that each of thesecomponents can also act as the over-voltage protection controller. Ifeither of these two is used in the over-voltage protection circuit, theuse of a resistor or other over-voltage protection device is optionalsince the zener diode and shunt resistor act as both over-voltageprotection controller and over-voltage protection device. In the case ofthe zener diode, a switch is preferably connected to the zener diode;the switch is closed during charging and open otherwise.

In a particular embodiment, an over-voltage protection circuit comprisesa shunt regulator (occasionally called a voltage reference) in serieswith a resistor, and these are in parallel with an energy storagedevice. The shunt regulator prevents the voltage of the energy storagedevice from rising above a set predetermined voltage and only allowscurrent to flow through it and the resistor when the voltage of theenergy storage device is at or above the predetermined voltage, therebyconserving power.

A capacitor and an inductor are both examples of an over-voltage devicethat temporarily stores excess charge drawn from the energy storagedevice. Since these devices do not generally dissipate charge, adissipation controller is preferably used in such configurations todissipate the excess charge stored in the over-voltage protectiondevice. The dissipation controller can comprise a dissipation switchthat connects the over-voltage protection device to ground in order todissipate the stored charge. Preferably, the dissipation controller alsocomprises a dissipation control mechanism that actuates the dissipationswitch so as to connect and disconnect the over-voltage protectiondevice from the dissipation controller.

There are alternative embodiments of the present invention that can beused in situations where the energy storage device comprises a pluralityof energy storage components. An over-voltage device that temporarilystores excess charge is advantageously used in such instances to avoidover-voltage by balancing charge between the plurality of energy storagecomponents.

Consider the exemplary case of a capacitor being used as an over-voltageprotection device for two energy storage components. The over-voltageprotection controller then comprises first and second switches coupledto the first and second energy storage components, respectively, andconnected in series to either end of the capacitor. The switches areactuated, during over-voltage, so as to connect or disconnect thecapacitor to each energy storage component in order to balance chargebetween them. One or more over-voltage detectors may be used in order todetect when over-voltage occurs. This detector can also control theactuation of the switches. Alternatively, an actuating means can beprovided that actuates the connection or disconnection of the capacitorto each energy storage component at a regular time interval.

Next, consider the exemplary case of an inductor being used as anover-voltage protection device for two energy storage components. Theover-voltage protection controller comprises first and second switchescoupled to the first and second energy storage components, respectively,and connected in series with the inductor with respect to the chargingcircuit. The over-voltage protection controller preferably furthercomprises first and second diodes connected in parallel with the firstand second switches, respectively. The switches are actuated, duringover-voltage, to connect or disconnect the inductor to each energystorage component in order to balance charge between them. One or moreover-voltage detectors may be used in order to detect when over-voltageoccurs. This detector can also control the actuation of the switches.Alternatively, an actuating means can be provided that actuates theconnection or disconnection of the capacitor to each energy storagecomponent at a regular time interval.

In another aspect of the invention, an over-voltage protection circuitis provided for connection to a charging circuit for use with a handhelddevice for maintaining a voltage across an energy storage device at orbelow a predetermined voltage so as to avoid over-voltage duringcharging. The circuit comprises an over-voltage protection device and anover-voltage protection controller. The over-voltage protectioncontroller connects the over-voltage protection device in parallel withthe energy storage device in response to an over-voltage condition atthe energy storage device, so as to draw excess charge from the energystorage device. The over-voltage protection circuit is connected tocharging leads, which are connected to the charging circuit.

Other aspects and features of the present invention will become apparentto those ordinarily skilled in the art upon review of the followingdescription of specific embodiments of the invention in conjunction withthe accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will now be described, by way ofexample only, with reference to the attached figures, wherein:

FIG. 1 illustrates a typical over-voltage protection circuit;

FIG. 2 illustrates a block diagram of an embodiment of the invention;

FIG. 3 illustrates an embodiment of the invention;

FIG. 4 illustrates an embodiment of the invention;

FIG. 5 illustrates an alternative embodiment of the invention; and

FIG. 6 illustrates a further alternative embodiment of the invention.

DETAILED DESCRIPTION

Generally, the present invention provides an over-voltage protectioncircuit for protection against over-voltage of an energy storage devicewhile charging. The circuit advantageously operates within theoperational limits of a battery-operated device, such as a mobile orhandheld device.

An over-voltage protection circuit according to the present inventioncomprises an over-voltage protection device, and an over-voltageprotection controller. The controller allows current to flow to theover-voltage protection device only when an energy storage device isexperiencing over-voltage. In allowing current to flow to theover-voltage protection device only when the voltage across the energystorage device is above a predetermined voltage, power conservation isachieved.

FIG. 2 illustrates an embodiment of the present invention in blockdiagram form. In particular, an over-voltage protection circuit 112 isillustrated, which is preferably connected in parallel to a chargingcircuit (not shown) via charging leads 110. The over-voltage protectioncircuit 112 is used during battery charging for maintaining a voltageacross an energy storage device, such as a battery, below apredetermined voltage. The over-voltage protection circuit 112 comprisesan over-voltage protection controller 114 and an over-voltage protectiondevice 116.

The over-voltage protection controller 114 can comprise any device thatis actuated in response to a voltage measured between nodes A and C thatmeets or exceeds a predetermined voltage, and connects the over-voltageprotection device 116 in parallel with the energy storage device 102when actuated. Consequently, current is conducted to the over-voltageprotection device 116 and charge drawn from the energy storage device102 only when over-voltage occurs. The over-voltage protectioncontroller 114 can comprise a switch, which is actuated in response to avoltage measured between nodes A and C that meets or exceeds thepredetermined voltage. When the switch is actuated in response to anover-voltage condition, current is conducted to the over-voltageprotection device 116 and charge is drawn from the energy storage device102. The switch can be, for example, a field effect transistor (FET),relay switch, bipolar junction transistor (BJT) or multiplexer (MUX).The switch preferably intrinsically comprises an over-voltage detectorthat causes the switch to be actuated when a voltage measured betweennodes A and C exceeds a predetermined voltage. Alternatively, a separateover-voltage detector can be used in conjunction with the switch.

The over-voltage protection device 116 draws charge from the energystorage device 102 experiencing over-voltage. The over-voltageprotection device 116 can comprise any device that is able to accept thedrawn excess charge and dispose of it. The over-voltage protectiondevice 116 can dissipate the energy itself, for example if a resistor isused. Alternatively, the over-voltage protection device 116 maytemporarily store the excess charge, for example if a capacitor orinductor is used, then transfer it elsewhere to be dissipated. In thelatter case, the over-voltage protection device 116 may temporarilystore such charge until it is connected to a dissipation controller (notshown), at which time the charge stored therein may be dissipated in anappropriate manner, as will be well known to one skilled in the art.

For example, the dissipation controller can comprise a circuit having adissipation switch that connects said over-voltage protection device toground in order to dissipate the stored voltage. Preferably, thisdissipation controller will also comprise a dissipation controlmechanism that actuates the dissipation switch so as to connect anddisconnect the over-voltage protection device 116 from the dissipationcontroller according to appropriate conditions.

Of course, the energy storage device 102 may, in fact, comprise aplurality of energy storage components connected in series. In such acase, a separate over-voltage protection circuit 112 can be connected inparallel to the terminals of each energy storage component in order toachieve a similar result as described in the embodiments above.

FIG. 3 illustrates a presently preferred embodiment of the presentinvention, showing an over-voltage protection circuit 112. In theembodiment shown in FIG. 3, a shunt regulator 118 is used as theover-voltage protection controller 114. The use of a shunt regulator isadvantageous in that shunt regulators have very sharp ‘turn-on’characteristics. Suppose, for example, that a shunt regulator is chosenwhose rated threshold voltage is below, but preferably near, the maximumspecified voltage of the energy storage device 102. When the voltagemeasured across the energy storage device 102 is below the thresholdvoltage of shunt regulator 118, negligible current will flow through theshunt regulator 118. If the voltage of the energy storage device 102rises during charging to the threshold voltage of the shunt regulator118, the shunt regulator causes current to flow through it and throughresistor 120. Any excess energy is dissipated primarily across theresistor 120, which is employed in this example as the over-voltageprotection device 116. Current continues to flow through the shuntregulator 118 until the voltage of the energy storage device 102 fallsbelow the threshold voltage of the shunt regulator. The shunt regulator118, therefore, acts as both a switch and an over-voltage detector inthis embodiment. If the leakage resistance in the shunt regulator 118 issuitable, the shunt regulator 118 can also perform the function of theover-voltage protection device 116, thereby obviating the need forresistor 120.

Although FIG. 3 illustrates an exemplary embodiment of the invention,many alternative embodiments are possible. The energy storage device 102may comprise a plurality of energy storage components connected inseries and a separate over-voltage protection circuit 112 can beconnected in parallel to the terminals of each energy storage componentin order to achieve a similar result as described in the embodimentsabove.

FIG. 4 illustrates an example of such an alternative embodiment. In FIG.4, a zener diode 122 can be used instead of the shunt regulator 118 asthe over-voltage protection controller 114. The zener diode 122 isadvantageously chosen such that its threshold voltage is equal to orslightly less than the predetermined voltage at or over whichover-voltage will occur. In this embodiment, it is preferable to have aswitch 124 disposed between the zener diode 122 and node C. The switch124 is closed during charging and open otherwise, so that power isdissipated in the zener diode 122 only while charging the energy storagedevice 102. As one skilled in the art can appreciate, little currentflows through the zener diode 122 as long as the voltage of the energystorage device 102 remains below the threshold voltage of the zenerdiode. If the voltage rises above the threshold voltage of the zenerdiode 122, exponentially greater current flows through the zener diode.Either the zener diode itself, or a combination of the zener diode 122and a series resistor or resistors (not shown in FIG. 4), dissipatesthis excess energy. In the case of using the zener diode 122 by itself,it is both over-voltage protection controller 114 and over-voltageprotection device 116. Where the zener diode 122 is used in conjunctionwith a resistor, or a plurality of resistors, (not shown in FIG. 4) theover-voltage protection device 116 functionally comprises both the zenerdiode 122 and the resistor.

There are, however, further alternative embodiments that may beconsidered in the case where the energy storage device comprises aplurality of energy storage components. Such embodiments cannot beimplemented with the energy storage device comprising only one energystorage component. FIGS. 5 and 6 illustrate such exemplary alternativeembodiments. Although these figures illustrate an energy storage devicecomprising two energy storage components, the designs may be employed incircuits having more than two energy storage components, withappropriate modifications being apparent to one of ordinary skill in theart. For example, if energy storage components are provided in multiplesof two, circuits such as those illustrated in FIGS. 5 and 6 may beconnected in parallel with each pair of energy storage components.Alternatively, the over-voltage protection device may be suitablyconnected to more than two energy storage components, as long as theproperties of the over-voltage protection device are selected such thatit can handle possible over-voltage from each of the energy storagecomponents to which it is connected.

FIG. 5 illustrates an alternative embodiment of the invention. In thisfigure, energy storage device 126 comprises the energy storagecomponents 102, 104, which are connected to the over-voltage protectioncircuit 112. In FIG. 5, the over-voltage protection controller 114comprises switches 128, 130, and the over-voltage protection device 116comprises capacitor 132. These switches 128, 130 could be, for example,FETs, relay switches, BJTs, MUXs, or any other suitable means asdescribed earlier. The switches 128, 130 are connected to a capacitor132 in order to protect against over-voltage by balancing the chargebetween the energy storage components. When the energy storagecomponents 102, 104 are charged and one energy storage component is ator above the predetermined voltage, the switches 128, 130 are actuatedand connect or disconnect the capacitor 132 to each energy storagecomponent 102, 104 in order to balance the charge between them.

In this embodiment, the switches 128, 130 are actuated in phase with oneanother as long as over-voltage occurs. Over-voltage detectors 134 and136 preferably control such actuation for switches 128, 130respectively. In this case, the over-voltage detector performs thefunctions of both detecting when over-voltage occurs, and controllingthe actuation of the switch. A single integral over-voltage detector canalternatively perform the functions of the two over-voltage detectors134 and 136.

An advantage of this embodiment is that any excessive charge istransferred from the energy storage component with greater charge to theenergy storage component with lesser charge and such excessive charge isnot dissipated as it is across the resistors in FIG. 1. For example, ifenergy storage component 102 were at or over the predetermined voltage,switches 128 and 130 would connect capacitor 132 in parallel to energystorage component 102, so that the charge is then transferred to thecapacitor 132. Later, switches 128 and 130 would connect capacitor 132to energy storage component 104 and charge would transfer to energystorage component 104 since its voltage is lower that that of energystorage component 102. Once again, the actuation of the switches ispreferably controlled as described above.

Alternatively, instead of using the over-voltage detectors 134 and 136,the circuit can comprise an actuating means (not shown in FIG. 5) thatactuates the connection and disconnection of the capacitor 132 to eachenergy storage component 102, 104 at a regular time interval. Thisprovides for automatic charge balancing without the need for theover-voltage detectors 134, 136.

In a case such as in FIG. 5 where the over-voltage protection device 116temporarily stores charge associated with drawn excess charge, adissipation controller 138 is preferably provided as part of theover-voltage protection circuit 114. This dissipation controller 138enables the charge stored in the over-voltage protection device 116 tobe dissipated in an appropriate manner, as will be well known to oneskilled in the art. For example, the dissipation controller can comprisea circuit having a dissipation switch that connects said over-voltageprotection device to ground in order to dissipate the stored voltage.Preferably, this dissipation controller will also comprise a dissipationcontrol mechanism that actuates the dissipation switch so as to connectand disconnect the over-voltage protection device 116 from thedissipation controller according to appropriate conditions. In analternative embodiment, a single integral controller may perform all thefunctions of over-voltage detectors 134, 136 as well as those of thedissipation controller 138.

FIG. 6 is another alternative embodiment of the invention. This figureillustrates a circuit that operates similarly to the circuit in FIG. 5,but has an improved efficiency over the embodiment in FIG. 5. In FIG. 6,energy storage device 126 comprises the energy storage components 102,104, which are connected to the over-voltage protection circuit 112.Each energy storage component 102, 104 is connected to switch 128, 130.The switches 128, 130 alternatively connect the respective energystorage component to an inductor 140, thus moving any excess chargebetween the energy storage components. In this embodiment, the switches128, 130 are actuated out of phase with one another and cannot both beclosed at the same time.

Diodes 142 and 144 conduct during the brief interval when one switch hasopened and the other has not yet closed while there is energy stored ininductor 140. When an energy storage component charges to (or just over)the predetermined voltage, the factor affecting which switch will closefirst is which energy storage component has the greater voltage.

For example, consider the situation where, within the circuit in FIG. 6,energy storage component 104 is at or just above the predeterminedvoltage and has a greater voltage than energy storage component 102.Then, switch 130 closes for a period of time to energize, but notsaturate, inductor 140. Later, switch 130 opens and diode 142immediately begins to conduct, because there is energy stored ininductor 140. Switch 128 subsequently closes, short-circuiting diode 142to improve efficiency, since switch 128 has a lower voltage across itthan diode 142 when it is closed, and thereby transferring charge toenergy storage component 102.

In FIG. 6, since the over-voltage protection device 116 temporarilystores charge associated with drawn excess charge, a dissipationcontroller 138 is preferably provided as part of the over-voltageprotection circuit 114. This dissipation controller 138 enables thecharge stored in the over-voltage protection device 116 to be dissipatedin an appropriate manner, as will be well known to one skilled in theart. For example, the dissipation controller can comprise a circuithaving a dissipation switch that connects said over-voltage protectiondevice to ground in order to dissipate the stored voltage. Preferably,this dissipation controller will also comprise a dissipation controlmechanism that actuates the dissipation switch so as to connect anddisconnect the over-voltage protection device 116 from the dissipationcontroller according to appropriate conditions. In an alternativeembodiment, a single integral controller may perform all the functionsof over-voltage detectors 134, 136 as well as those of the dissipationcontroller 138.

The above-described embodiments of the present invention are intended tobe examples only. Alterations, modifications and variations may beeffected to the particular embodiments by those of skill in the artwithout departing from the scope of the invention, which is definedsolely by the claims appended hereto.

1. An over-voltage protection circuit for connection to a chargingcircuit for maintaining a voltage across a plurality of electricalenergy storage devices at or below a predetermined voltage duringcharging, comprising: a first electrical energy storage device connectedfrom a top voltage rail to a primary junction; a second electricalenergy storage device connected from the primary junction to a bottomvoltage rail, the first electrical energy storage device and the secondelectrical energy storage device charged by current flowing from the toprail through the first electrical energy storage device and the secondelectrical energy storage device to the bottom rail; an inductor; and afirst switch and a second switch; the over-voltage protection circuit toperform a first procedure in which a charge is drawn from the firstelectrical energy storage device through the inductor to energize theinductor, the inductor then to cause a current that charges the secondstorage device, the first procedure beginning when the first switch isclosed; the over-voltage protection circuit to perform a secondprocedure in which a charge is drawn from the second electrical energystorage device through the inductor to energize the inductor, theinductor then to cause a current that charges the first storage device,the second procedure beginning when the second switch is closed; theinductor to be energized to less than saturation level; a voltagesensor, the voltage sensor to close the first switch in response to thefirst electrical energy storage device being excessively charged and toclose the second switch in response to the second electrical energystorage device being excessively charged; and a dissipation controllerto perform a third procedure in which a charge is discharged from theenergized inductor upon detection of an over-voltage condition by thevoltage sensor.
 2. The over-voltage protection circuit of claim 1, claim2, further comprising: a first diode, the first diode and the firstswitch connected from the top rail to a secondary junction; a seconddiode, the second diode and the second switch connected from thesecondary junction to the bottom rail, with the inductor connected fromthe primary junction to the secondary junction; the over-voltageprotection circuit being configured in the first procedure to: close thefirst switch to conduct current from the top rail, through the firstswitch and the inductor, to the primary junction to energize theinductor; and open the first switch, causing the inductor to induce atemporary current to flow from the bottom rail through the second diodeand the inductor, to the primary junction; and the over-voltageprotection circuit being configured in the second procedure to: closethe second switch to conduct current from the primary junction, throughthe second switch and the inductor, to the bottom rail to energize theinductor; and open the second switch, causing the inductor to induce atemporary current to flow from the primary junction through the firstdiode and the inductor, to the top rail.
 3. The circuit of claim 1,wherein the first electrical energy storage device is a capacitor. 4.The circuit of claim 1, wherein the first electrical energy storagedevice is a battery.
 5. An over-voltage protection circuit forconnection to a charging circuit for maintaining a voltage across aplurality of electrical energy storage devices at or below apredetermined voltage during charging, comprising: a first electricalenergy storage device connected from a top voltage rail to a primaryjunction; a second electrical energy storage device connected from theprimary junction to a bottom voltage rail, the first electrical energystorage device and the second electrical energy storage device chargedby current flowing from the top rail through the first electrical energystorage device and the second electrical energy storage device to thebottom rail; an inductor; a first switch and a second switch; and adissipation controller to perform a dissipation procedure in which acharge is discharged from the energized inductor upon detection of anover-voltage condition by a voltage sensor; the over-voltage protectioncircuit to perform a first procedure in which a charge is drawn from thefirst electrical energy storage device through the inductor to energizethe inductor, the inductor then to cause a current that charges thesecond electrical energy storage device, the first procedure beginningwhen the first switch is closed; and the over-voltage protection circuitto perform a second procedure in which a charge is drawn from the secondelectrical energy storage device through the inductor to energize theinductor, the inductor then to cause a current that charges the firstelectrical energy storage device, the second procedure beginning whenthe second switch is closed.
 6. The over-voltage protection circuit ofclaim 5, further comprising: a first diode, the first diode and thefirst switch connected from the top rail to a secondary junction; asecond diode, the second diode and the second switch connected from thesecondary junction to the bottom rail, with the inductor connected fromthe primary junction to the secondary junction; the over-voltageprotection circuit being configured in the first procedure to: close thefirst switch to conduct current from the top rail, through the firstswitch and the inductor, to the primary junction to energize theinductor; and open the first switch, causing the inductor to induce atemporary current to flow from the bottom rail through the second diodeand the inductor, to the primary junction; and the over-voltageprotection circuit being configured in the second procedure to: closethe second switch to conduct current from the primary junction, throughthe second switch and the inductor, to the bottom rail to energize theinductor; and open the second switch, causing the inductor to induce atemporary current to flow from the primary junction through the firstdiode and the inductor, to the top rail.
 7. The over-voltage protectioncircuit of claim 5, wherein the voltage sensor closes the first switchin response to the first storage device being excessively charged, andcloses the second switch in response to the second storage device beingexcessively charged.
 8. A method of operating an over-voltage protectioncircuit electrical circuit for charging a plurality of electrical energystorage devices that includes a first electrical energy storage deviceconnected from a top voltage rail to a primary junction, a secondelectrical energy storage device connected from the primary junction toa bottom voltage rail, the first electrical energy storage device andthe second electrical energy storage device charged by current flowingfrom the top rail through the first electrical energy storage device andthe second electrical energy storage device to the bottom rail, aninductor, a first diode and a first switch, the first diode and thefirst switch connected from the top rail to a secondary junction, and asecond diode and a second switch, the second diode and the second switchconnected from the secondary junction to the bottom rail, with theinductor connected from the primary junction to the secondary junction,the method comprising: performing a first procedure in which a charge isdrawn from the first electrical energy storage device through theinductor to energize the inductor, the inductor then to cause a currentthat charges the second electrical energy storage device; the firstprocedure including: closing the first switch to conduct current fromthe top rail, through the first switch and the inductor to the primaryjunction to energize the inductor; and opening the first switch to causethe inductor to induce a temporary current to flow from the bottom railthrough the second diode and the inductor to the primary junction; andperforming a second procedure in which a charge is drawn from the secondelectrical energy storage device through the inductor, the inductor thento cause a current that charges the first electrical energy storagedevice; the second procedure including: closing the second switch toconduct current from the primary junction, through the inductor and thesecond switch, to the bottom rail to energize the inductor; and openingthe second switch to cause the inductor to induce a temporary current toflow from the primary junction through the inductor and the first diodeto the top rail; the over-voltage protection circuit being configured toperform the first procedure in response to the first electrical energystorage device being excessively charged and to perform the secondprocedure in response to the second electrical energy storage devicebeing excessively charged; and discharging the energized inductor upondetection of an over-voltage condition.
 9. The method of claim 8,wherein the first electrical energy storage device is a capacitor. 10.The method of claim 8, wherein the first electrical energy storagedevice is a battery.